Replace virtualenv by python3 -m venv, fix #48

[squalou]

I reported in this fork the tests I did on my test env, everything seems to work fine. Some already installed clients updated just fine and continued to work with venv previouly created with virtualenv.

Documentation may require a bit of attention.

Also I have not tested Arch, according to doc there's no -dev package so python headers are included ... but sot user pip will compile things all right.

I can get my hands on a arch VM to make a quick test I think.